1. Field of the Invention
The present invention relates to the forming of electronic printed circuits of the thick-layer hybrid type, to certain means adapted for carrying out the forming process and to the printed circuits formed thereby
2. Description of the Prior Art
Hybrid microelectronics has evolved in the art in response to two concerns: to solve the problems of bulk and to improve the reliability of the circuits. As distinct from a monolithic circuit, which is obtained from a semiconducting substrate in which all of the components (active and passive) are simultaneously produced in the course of but a single process, a hybrid circuit is produced on an insulating substrate and contains only passive elements; if there are any active elements, namely, semiconductors or integrated circuits, same are later added by soldering. Hybrid circuits are classified as thin-layer circuits and thick-layer circuits. This classification relates to the thickness of the deposited layers: from 0.02 to 10 .mu.m in the case of thin layers, and from 10 to 50 .mu.m in the case of thick layers.
For the thin layers, the layers are typically deposited by utilizing the techniques of vacuum evaporation or cathodic sputtering. For the thick-layer hybrids, the technology used generally consists of printing the desired circuits directly onto the insulating substrate by means of conventional silk-screen processes.
Silk-screen printing consists of forcing the ink which it is desired to deposit onto the substrate through the fine meshes of a screen, some of which are blocked by a special lacquer, the free meshes being a very precise representation of the half-tone drawing of the circuit to be reproduced.
Any hybrid circuit must be produced on a substrate. The final quality of the circuit will depend upon the selection of a good substrate. The substrate generally serves a triple purpose: that of mechanical support, not only for the deposited circuit but also for the active components which it is to receive; that of electrical insulator, its resistivity, therefore, having to be as high as possible and its loss factor as low as possible; and that of heat dissipator, which therefore requires an excellent thermal conductivity and a high specific heat. Among the suitable substrates, representative are: inorganic materials, such as, for example, ceramics like beryllium oxide, alumina and Pyrex glass; and organic materials, such as, for example, reinforced resins made from thermosetting polymeric substances.
Once it has received a conducting, insulating or resistant print in the form of an ink having a well-defined rheology, the substrate is baked or cured in an oven such that the deposit becomes perfectly integral with the substrate. Finally, the output wires are attached and, after connection of the active components, if appropriate, the finished module can be coated by encapsulation in a glass or a polymeric resin in order to protect it from the action of moisture and to make it easier to handle.
Virtually all thick-layer circuits each receive several successive prints. In a typical process, the conducting layer is first printed and baked; the resistors, which well be adjusted to the required values, are then printed and baked. This process can also include the printing of an insulating layer. It is thus possible to successively deposite, for example, conducting insulating, conducting and resistant layers. The stages of the process are summarized in FIG. 8.
As regards the silk-screen printing inks required for the manufacture of the thick-layer circuits immediately above-described, these are therefore of three types: conducting, resistant and insulating inks. The conducting and resistant inks typically consist of the combination of a metal extender with a binder and, if appropriate, a diluent making it possible to adjust the rheological properties of the inks. By modifying the chemical structure of the binder, the baking fixes the metal deposit to the substrate and makes it possible to obtain the desired final conducting or resistant circuit. The insulating inks differ from the aforesaid inks essentially by the fact that they do not contain conducting metal extenders.
In hybrid circuits, the ink binder commonly consists of inorganic particles, such as, for example, fusible special glasses in powder form. As these glass-containing inks are baked at about 700.degree. C. to 1,100.degree. C., their use means that ceramics are preferably selected as the insulating substrates, and powders of non-oxidizable noble metals, in particular platinum, gold, silver or palladium/gold, palladium/silver or platinum/gold alloys, or powders of conducting oxides derived from noble metals, are selected as the conducting metal extenders. Because of the use of noble metals, the adoption of this technique results in high expenditure, which will constitute an obstacle to its development. It too has been proposed to use powders of the non-noble metals, such as, for example, copper, but in that case the final baking should be carried out in a non-oxidizing atmosphere, and this measure complicates the technology of the ovens which can be used.
It is possible to replace the aforesaid inorganic binder by an organic binder, such as, for example, an epoxy resin. The ink-baking temperatures required in this case are considerably lower than in the case of the inorganic binders; same typically range from 100.degree. C. to 300.degree. C. It is then possible to use types of insulating substrates other than ceramics, such as, for example, reinforced organic resins, and to use inks based upon a non-noble metal extender, there being greatly reduced risks of oxidation of the metal extender during baking carried out, in the conventional manner, in the atmosphere at from 100.degree. C. to 300.degree. C. However, the inks prepared and baked in this manner entail a unique and significant disadvantage: they are difficult to solder. Apart from their function of conducting an electric current, the inks must in fact be contact points for receiving wires or, if appropriate, lugs of active components, which assumes an excellent suitability for soldering, in particular with tin-based alloys.
In the metallization of insulating substrates, to produce high-quality conducting printed circuits capable of withstanding soldering without difficulty, it has been proposed to use inks comprising an organic binder made of a polymeric substance and a metal extender based on cuprous oxide (compare U.S. Pat. Nos. 3,226,256 and 3,347,724). After baking, the deposit obtained is reduced by treatment with an acid, in order to convert the cuprous oxide to copper metal; this layer of copper is then reinforced by deposition of a ductile metal, which can also be copper, employing a chemical oxidation/reduction reaction. It is known that the reduction of cuprous oxide with an acid involves the formation of an unstable cuprous salt which disproportionates to give, on the one hand, cupric salts, and, on the other hand, copper metal, which deposits, according to the equation: EQU Cu.sub.2 O+2H.sup.+ .fwdarw.Cu.sup.++ +Cu+H.sub.2 O
In a reduction of this type, the yield of copper metal is well below 50%: in fact only half of the initial copper is capable of being converted to conducting metal and, furthermore, a portion of this reduced copper is dissolved by the acid agent used. As a result, the metal deposit obtained will only be very slightly conducting. It is for this reason that, in this technique, recharging of the deposited circuit with metal is always carried out after the reduction step. The fact that such recharging is carried out by a chemical oxidation/reduction method and not by an electrolytic method (which does not work) is confirmation of the poorly conducting nature of the original deposit. A first disadvantage of this method is the fact that the recharging with metal is a lengthy operation, the deposition rate in fact being on the order of 1 .mu.m per hour. A second disadvantage is the fact that there will be an increase in the thickness of the original deposit, which may reach 100%, and this increase in the thickness is likely to limit the number of layers deposited by silk-screen printing, in order to avoid an excessively great loss of inherent flatness of the substrate as a result of the successive depositions.
Thus, it has proved necessary to develop, in the manufacture of hybrid circuits, a technique by means of which conducting circuits could be economically deposited and in a technically simple manner, without electrochemical recharging, the said conducting circuits being suitable for soldering and not containing extra thicknesses.
Furthermore, as has been explained above, the thick-layer hybrid circuits each receive several successive silk-screen prints. For example, a condenser will be formed by the deposition and baking of the lower electrode 1, the printing and baking of the dielectric 2 and the printing of the upper electrode 3, followed by baking of the assembly (compare FIG. 1 of the drawings, which represents a condenser in section). The manufacturing process therefore involves three successive depositions with two different inks: a conducting ink and an insulating ink. For other types of hybrid circuits, the manufacturing process may also involve additional depositions with a resistant ink. Here again, it proved necessary to develop a technique making it possible to simplify the procedure for printing the layers, in particular by limiting the number of inks required.